The root sections of fan blades currently operative in turbofan engines around the world, employ straight root attachments in which the mean lines of each blade root and of the corresponding attachment slot extend diagonally across the rim of the disk. Such designs have heretofore proved adequately successful in securing the blades to the disk. Future turbofan engines, however, are likely to incorporate fan blades subjected to higher tip speeds, and resultantly greater restraining forces need be applied at the root sections. In such engines much attention need be given to concentrated stress levels along the span of the fan blades, such as occurs at conventionally integrated flow confining platforms and at the root sections. It is the reduction of such concentrated stress levels to which the present concepts are directed.
Although a novel structure is taught herein, some prior constructions bearing geometric similarity to the fan blades of the present invention are found in the steam turbine art field and in the gas turbine art field. Representative illustrations in the steam turbine field include those shown in U.S. Pat. Nos. 1,041,269 to Guyer entitled "Blading for Turbines"; 1,719,415 to Back entitled "Turbine Blade Attachment"; 1,793,468 to Densmore entitled "Turbine Blade"; and 3,986,793 to Warner et al entitled "Turbine Rotating Blade". Each of the patents discloses turbine blades having root sections formed to arcuate geometries so as to be proximately contoured to the respective airfoils. In each instance the root sections are of uniform cross section geometry across the attachment.
Within the gas turbine engine field, a geometrically similar but technically distinct construction is disclosed in French Pat. No. 1,143,952. By the disclosed structure, an arcuated root geometry is applied to a cooled turbine blade for the purpose of enabling cooling air to flow into a multi-passage blade. In one turbine structure of the French patent the roots of the blades are flared so as, according to the reference, to be held firmly into the turbine assembly by a wedging action. Note, however, that the expected aerodynamic loads on the blade are in a direction tending to unseat the wedged root. Additionally, note that the center of mass of the blade is located over a region of comparatively smaller root cross sections.
Notwithstanding the above described disclosures of the curved and/or tapered roots of turbine blades, fan blade roots have remained configured to straight geometries.